Pattern Formation
Many types of communication take place among animal cells. These vary from long-range effects, such as those of rather stable hormones circulating in the blood and acting on any cells in the body that possess the appropriate receptors, however distant they are, to the fleeting effects of very unstable neurotransmitters operating over distances of only a few microns. Of particular importance in development is the class of cell interactions called embryonic induction; this includes influences operating between adjacent cells or in some cases over greater than 10 cell diameters (Saxen et al. (1989) Int J Dev Biol 33:21-48; and Gurdon et al. (1987) Development 99:285-306). Embryonic induction is defined as in interaction between one (inducing) and another (responding) tissue or cell, as a result of which the responding cells undergo a change in the direction of differentiation. This interaction is often considered one of the most important mechanism in vertebrate development leading to differences between cells and to the organization of cells into tissues and organs. Adult organs in vertebrates, and probably in invertebrates, are formed through an interaction between epithelial and mesenchymal cells, that is, between ectoderm/endoderm and mesoderm, respectively.
The effects of developmental cell interactions are varied. Typically, responding cells are diverted from one route of cell differentiation to another, by inducing cells that differ from both the uninduced and induced states of the responding cells (inductions). Sometimes cells induce their neighbors to differentiate like themselves (homoiogenetic induction); in other cases a cell inhibits its neighbors from differentiating like itself. Cell interactions in early development may be sequential, such that an initial induction between two cell types leads to a progressive amplification of diversity. Moreover, inductive interactions occur not only in embryos, but in adult cells as well, and can act to establish and maintain morphogenetic patterns as well as induce differentiation (J. B. Gurdon (1992) Cell 68:185-199).
Pattern formation is the activity by which embryonic cells form ordered spatial arrangements of differentiated tissues. The physical complexity of higher organisms arises during embryogenesis through the interplay of cell-intrinsic lineage and cell-extrinsic signaling. Inductive interactions are essential to embryonic patterning in vertebrate development from the earliest establishment of the body plan, to the patterning of the organ systems, to the generation of diversive cell types during tissue differentiation (Davidson, E., (1990) Development 108:365-389; Gurdon, J. B., (1992) Cell 68:185-199; Jessell, T. M. et al., (1992) Cell 68:257-270). The effects of developmental cell interactions are varied. Typically, responding cells are diverted from one route of cell differentiation to another by inducing cells that differ from both the uninduced and induced states of the responding cells (inductions). Sometimes cells induce their neighbors to differentiate like themselves (homoiogenetic induction); in other cases a cell inhibits its neighbors from differentiating like itself. Cell interactions in early development may be sequential, such that an initial induction between two cell types leads to a progressive amplification of diversity. Moreover, inductive interactions occur not only in embryos, but in adult cells as well, and can act to establish and maintain morphogenetic patterns as well as induce differentiation (J. B. Gurdon (1992) Cell 68:185-199).
The origin of the nervous system in all vertebrates, for example, can be traced to the end of gastrulation. At this time, the ectoderm in the dorsal side of the embryo changes its fate from epidermal to neural. The newly formed neuroectoderm thickens to form a flattened structure called the neural plate which is characterized, in some vertebrates, by a central groove (neural groove) and thickened lateral edges (neural folds). At its early stages of differentiation, the neural plate already exhibits signs of regional differentiation along its anterior posterior (A-P) and mediolateral axis (M-L). The neural folds eventually fuse at the dorsal midline to form the neural tube which will differentiate into brain at its anterior end and spinal cord at its posterior end. Closure of the neural tube creates dorsal/ventral differences by virtue of previous mediolateral differentiation. Thus, at the end of neurulation, the neural tube has a clear anterior-posterior (A-P), dorsal ventral (D-V) and mediolateral (M-L) polarities (see, for example, Principles in Neural Science (3rd), eds. Kandel, Schwartz and Jessell, Elsevier Science Publishing Company: NY, 1991; and Developmental Biology (3rd), ed. S. F. Gilbert, Sinauer Associates: Sunderland Mass., 1991). Inductive interactions that define the fate of cells within the neural tube establish the initial pattern of the embryonic vertebrate nervous system. In the spinal cord, the identity of cell types is controlled, in part, by signals from two midline cell groups, the notochord and floor plate, that induce neural plate cells to differentiate into floor plate, motor neurons, and other ventral neuronal types (van Straaten et al. (1988) Anat. Embryol. 177:317-324; Placzek et al. (1993) Development 117:205-218; Yamada et al. (1991) Cell 64:035-647; and Hatta et al. (1991) Nature 350:339-341). In addition, signals from the floor plate are responsible for the orientation and direction of commissural neuron outgrowth (Placzek, M. et al., (1990) Development 110:19-30). Besides patterning the neural tube, the notochord and floorplate are also responsible for producing signals which control the patterning of the somites by inhibiting differentiation of dorsal somite derivatives in the ventral regions (Brand-Saberi, B. et al., (1993) Anat. Embryol. 188:239-245; Porquie, O. et al., (1993) Proc. Natl. Acad. Sci. USA 90:5242-5246).
Another important signaling center exists in the posterior mesechyme of developing limb buds, called the Zone of Polarizing Activity, or “ZPA.” When tissue from the posterior region of the limb bud is grafted to the anterior border of a second limb bud, the resultant limb will develop with additional digits in a mirror-image sequence along the anteroposterior axis (Saunders and Gasseling, (1968) Epithelial-Mesenchymal Interaction, pp. 78-97). This finding has led to the model that the ZPA is responsible for normal anteroposterior patterning in the limb. The ZPA has been hypothesized to function by releasing a signal, termed a “morphogen”, which forms a gradient across the early embryonic bud. According to this model, the fate of cells at different distances from the ZPA is determined by the local concentration of the morphogen, with specific thresholds of the morphogen inducing successive structures (Wolpert, (1969) Theor. Biol. 25:1-47). This is supported by the finding that the extent of digit duplication is proportional to the number of implanted ZPA cells (Tickle, (1981) Nature 254:199-202).
In principle, induction means any process in which the developmental pathway of one cell population is controlled by signals emitted from another. For instance, embryonic inductive signals are key regulatory proteins that function in vertebrate pattern formation, and are present in important signaling centers known to operate embryonically to define the organization of the vertebrate embryo. For example, these signaling structures include the notochord, a transient structure which initiates the formation of the nervous system and helps to define the different types of neurons within it. The notochord also regulates mesodermal patterning along the body axis. Another distinct group of cells having apparent signaling activity is the floorplate of the neural tube (the precursor of the spinal cord and brain) which also signals the differentiation of different nerve cell types. It is also generally believed that the region of mesoderm at the bottom of the buds which form the limbs (called the “Zone of Polarizing Activity” or “ZPA”) operates as a signaling center by secreting a morphogen which ultimately produces the correct patterning of the developing limbs. Moreover, inductive signals are required for cell differentiation and morphogenesis throughout vertebrate development. Thus, in addition to initiating differences between cells in early development, inductive signals are also involved in formation and maintenance of most, if not all, adult organs and tissues.
Growth factors are substances, such as polypeptide hormones, which affect the growth of defined populations of animal cells in vivo or in vitro, but which are not nutrient substances. Proteins involved in the growth and differentiation of tissues may promote or inhibit growth, and promote or inhibit differentiation, and thus the general term “growth factor” includes cytokines, trophic factors, and their inhibitors. Among growth, or neurotrophic, factors presently known are the transforming growth factors (TGF-alpha, TGF-beta, TGF-gamma). Transforming growth factor-beta appears to elicit a variety of responses in many different cell types.
Widespread neuronal cell death accompanies normal development of the central and peripheral nervous systems. Studies of peripheral target tissues during development have shown that neuronal cell death results from the competition among neurons for limiting amounts of survivor factors (“neurotrophic factors”). The earliest identified of these, nerve growth factor (“NGF”), is the most fully characterized and has been shown to be essential for the survival of sympathetic and neural crest-derived sensory neurons during early development of both chick and rat. Barde et al., U.S. Pat. No. 5,229,500, issued Jul. 20, 1993, describe nucleic acid sequences encoding brain derived neurotrophic factor (“BDNF”), as well as the BDNF protein. BDNF is suggested for treating Parkinson's Disease and Alzheimer's Disease. Additional uses (quite recently performed successfully) are for the identification of homologous regions between BDNF and NGF so as to identify and isolate additional members of the NGF family, and also to generate immunogen by antibodies directed toward BDNF or fragments.
Among TGF-beta members are the bone morphogenetic proteins (BMP). The BMPs have been indicated as useful in wound healing, tissue repair, and to induce cartilage and/or bone growth. For example, PCT Application 9309229, inventors Israel and Wolfman, published May 13, 1993, describes uses of proteins with bone stimulating activity such as bone fracture healing and possibly the treatment of periodontal disease and other tooth repair processes.
BMPs have potent effects during embryogenesis. One member, BMP-4, has been shown to have potent ventralizing effects in Xenopus embryos, leading to the differentiation of blood and mesenchyme and inhibiting the formation of dorsal tissues such as notochord, muscle, and nervous system. (See, e.g., Jones et al., Development, 115, pp. 639-647, 1991.) BMP-4 is expressed ventrally in the Xenopus embryo and its expression is increased by ventralizing treatments such as irradiation with ultraviolet light (UV), see Steinbeisser et al., EMBO J., in press, November 1994 issue. An inhibitor of ventralizing BMPs could have dorsalizing effects on tissue differentiation. There are precedents for such inhibitory interactions in the TGF-beta family, since activin, a dorsalizing factor, can be inhibited by a specific inhibitory protein designated inhibin in the Xenopus embryo (see, e.g., Hemmati-Brivanlou et al., Cell, 77, pp. 283-295, 1994).
Another family of neurotropic factors are the Wnts, which have dorsal axis-inducing activity. Most of the Wnt protein are bound to cell surfaces (see, e.g., Sokol et al., Science, 249, pp. 561-564, 1990). One member of the family, Xwnt-8, was described as to dorsal axis-inducing activity in Xenopus embryos by Smith and Harland in 1991, Cell, 67, pp. 753-765. The authors described using RNA injections as a strategy for identifying endogenous RNAs involved in dorsal patterning to rescue dorsal development in embryos that were ventralized by UV irradiation.
UV ventralization is useful to probe the normal response of a gene to dorsal/ventral cell identity because UV treated embryos reproducibly lack obvious dorsal structures (e.g., somites, notochord, and neural plate). In addition, gastrulae that has become extreme ventralized tadpoles form a radial blastopore lip at the time of normal ventral blastopore lip formation. This suggests that the mesoderm is behaving as though it is ventral in identity. Lithium chloride treatment respecifies the fate of cells along the anterior-posterior axis of the early embryo by contrast to UV irradiation, which causes centralization of embryos.
Identification of new proteins, polypeptides or compounds capable of modulating embryonic patterning and cellular differentiation would aid in the development of therapeutic treatments for a wide variety of conditions involving aberrant cellular proliferation.
The Aryl Hydrocarbon Receptor
The Aryl Hydrocarbon (Ah) receptor is an intracellular cytosolic protein found in higher vertebrates in several epithelial tissues. The effects of Ah receptor ligands are known almost entirely in regards to their effects on P4501A1 induction, an enzyme system that metabolizes certain xenobiotics (Landers and Bunce, 1991). The Ah receptor was discovered by Poland and co-workers and studied first as a high affinity binding protein for aryl hydrocarbons of toxicological importance, most notably 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) (Poland et al., 1976).
Dioxins or dioxin-like compounds are environmental pollutants produced as unwanted byproducts of common industrial processes such as paper bleaching, incineration and chemical manufacturing.
Dioxins or dioxin-like compounds are a loosely defined family of organochlorine molecules with close structural and chemical similarities. Additionally, these compounds, by virtue of their similar structure and chemistry, share a common mechanism of toxicity. The prototypical dioxin, and the best studied, is 2,3,7,8 Tetrachlorodibenzo-P-Dioxin (sometimes called 2,3,7,8-TCDD or TCDD or dioxin). Besides 2,3,7,8 Tetrachlorodibenzo-P-Dioxin, this group of compounds include not only the dibenzo-p-dioxins, but also dibenzofurans, azobenzenes, dibenzo-ethers, certain polychlorinated biphenyls, certain polyaromatics and other compounds. Toxicity of these compounds is dependent on a planar, polyaromatic structure with lateral halogen substitutions.
The biochemical and physiological basis of dioxin toxicity has been the subject of intense scientific scrutiny. Animals vary in their susceptibility to dioxins and in their symptoms. In guinea pigs, as little as 600 ng per kg produces a lethal wasting syndrome. In humans, toxic responses to dioxin exposure include several proliferative aberrations such as hyperkerotinosis and hyperplasia. Despite much research in the area, the biochemical and physiological events that produce toxicity are poorly understood.
Although the ultimate physiological events that produce toxicity are poorly understood, it is generally agreed that toxicity of these chemically and structurally related dioxin-like compounds is due to their ability, by virtue of their chemical and structural properties, to bind to the intracellular Ah receptor. Although the ability of a compound to be a ligand of the Ah receptor is a requirement for dioxin-like toxicity, these compounds must also be able to promote transformation of the receptor to a DNA-binding form subsequent to ligand binding in order to be toxic. The transformation of the Ah receptor comprises a series of poorly understood events that include dissociation of the inactive receptor from a complex of proteins that include one or more molecules of the chaperonin HSP90, the formation of a new complex that includes HSP90-dissociated Ah receptor plus bound dioxin and the nuclear protein Aryl Hydrocarbon Nuclear Translocator (ARNT), and the binding of the Ah receptor/ARNT complex to specific DNA sequences.
These sequences, called Dioxin-Response Elements (DREs) or Xenobiotic-Response Elements (XREs), lie upstream of the promoter regions of certain genes, the most studied being the P4501A1 gene. The binding of the transformed Ah receptor and associated protein(s) to the DREs enhance transcription of the associated genes. The inappropriate expression of these genes are thought to be the early events in the pleiotropic response to dioxins. It is fundamental that dioxins, in order to be toxic, must be able to both bind to the Ah receptor and transform it into an active form, and that this binding/transformation couplet is the central and only defined biochemical event in the toxic effects of dioxins.
Different dioxin-like compounds, although they share a common mechanism of toxicity, have different toxic potencies that can differ by several orders of magnitude. The toxicity of an unknown mixture of dioxin-like compounds can vary considerably depending on the identity and concentrations of the congeners present. Thus, the concept of Toxic Equivalency Factors (TEFs) and Toxic Equivalence (TEQs) have been advanced by some scientists. TEFs are the fractional toxicity of a dioxin-like compounds compared to the most toxic, prototypical 2,3,7,8-TCDD. Published TEFs are arbitrarily assigned values based on consensus toxicity's in the scientific literature. TEQs are the estimated toxic potential of a mixture of these compounds calculated by adding their respective TEFs with adjustment for their respective concentrations. TEFs and TEQs have been promoted by the EPA in order to facilitate their risk and hazard assessment of these compounds when they occur as mixtures.
The sequence of known events when an agonist or Ah ligand binds to the Ah receptor can be summarized as follows. The Ah receptor in the unbound state is found bound to the chaperonin HSP90 and another poorly understood protein or proteins (Perdew and Hollenback, 1990). Agonists of the Ah receptor such as TCDD, upon binding to the receptor, alter the receptor (commonly referred to as “transformation”) so that the liganded Ah receptor separates from the chaperonin complex, translocates to the nucleus, binds to the ARNT protein, binds to specific DNA sequences upstream of the P4501A1 gene sequence as the Ah receptor: ARNT complex, and enhances transcription of P4501A1.
Antagonists and inhibitors of the Ah receptor have not been well-studied. Research interest has focused on potent, toxic agonists of the Ah receptor such as TCDD. Research interest on antagonists of the Ah receptor has focused on understanding the biochemistry of the Ah receptor, interactions among man-made toxins, and as inhibitors of estrogen-mediated gene expression. Known antagonists of the Ah-receptor include some flavone derivatives (Gasiewicz and Rucci, 1991; and Lu et al., 1995) and synthetic aryl hydrocarbons (Merchant and Safe, 1995).
Ah receptor agonists and antagonists of plant and dietary origin are known (Kleman et al., 1994; Bjeldanes et al., 1991; and Jellinck et al., 1993). Interestingly, these compounds are thought to be anti-carcinogens, tumor promoters, or both, however, mechanisms of action remain unknown.
The biochemical effects of agonists of the Ah receptor are generally thought to be Ah receptor-dependent, that is, the potency of the toxic response is proportional to their ability transform the Ah receptor (Wheelock et al., 1996), or induce P4501A1 (Zacharewski et al., 1989). However, the induction of P4501A1 itself is probably not connected with most of the physiological effects of Ah receptor ligands. Ah receptor ligands can act as anti-estrogenic tumor dependent agents by virtue of the ability of the Ah receptor: ARNT complex to interfere with estrogen receptor-mediated transcription. TCDD effects on both cellular proliferation, and apoptosis may occur via perturbation of intracellular signal transduction systems involved with cellular proliferation and apoptosis, as evidenced with by intracellular protein phosphorylation (Ma, 1992), induction of protein-tyrosine kinases, and cyclin dependent kinases (Ma and Babish, 1993).
The natural function of the Ah receptor is unknown, however, deletion of the Ah receptor results in liver abnormalities and immune system impairment. Furthermore, the identification of any endogenous ligand has remained elusive, and how Ah receptor-mediated signaling interacts with cell cycle and apoptotic control is poorly understood, and a direct connection has not been established.